The present invention relates generally to a method for evaluating fuels, food products and oils, and, more specifically, to a method for analyzing the freshness of fuels, food products, oils, and any organic material or foodstuff which is subject to oxidative degradation, or for which oxidative degradation is a concern.
Fuels, such as jet fuel, gasoline, diesel fuel and kerosene, are typically stored for a period before use. During bulk storage and storage in engine tanks or other use containers prior to use, fuels undergo varying degrees of oxidation. Such fuels contain substantially no natural antioxidants to prevent or control oxidation. Antioxidant additives for fuels remain in development. Accordingly, at the present time, oxidation of most fuels takes place to varying degrees. During oxidation, typically peroxides and hydroperoxides act as oxidation initiators, and lead to production of aldehydes and additional peroxides and hydroperoxides, generally referred to as oxidation products, in the fuel. Other oxidation products, such as phenols, may also be produced. If allowed to go undetected, these oxidation products will lead to gum formation and fuel line clogging.
Similarly, food products, such as milk and milk products, meat products, fish, and dried fruits and vegetables, are subject to oxidation during storage. However, most natural food products include natural antioxidants. Milk, for example, includes tocopherols, essentially vitamin E, as an antioxidant which protects its freshness. The tocopherols, as antioxidants, neutralize oxidation initiators. As the tocopherols deplete over time to low levels, the milk remains consumable. After the natural antioxidants deplete, oxidative degradation of the milk produces aldehydes and peroxides and hydroperoxides, which eventually effect the taste and result in spoilage.
Tocopherols are also present in other food products, such as fish, dried fruits, and vegetables. Meat products contain natural antioxidants such as ascorbic acid and phospholipids. To preserve freshness, artificial antioxidants are often added to food products. Butylated hydroxy toluene (BHT) and butylated hydroxy anisole (BHA) are common.
Regardless, once oxidative degradation of organic materials begins, peroxides and hydroperoxides act as oxidation initiators, and produce additional peroxides and hydroperoxides and aldehydes, which result in spoilage. The process of degradation goes further in some organic materials, particularly those subjected to thermal cycles. In those materials, aldehydes are produced as intermediate products which further oxidize to produce carboxylic acids. Once oxidative degradation begins, food products become increasingly dangerous to consume, and illness may result in varying degrees.
Oils, lubricants and other fluids are often used in ways that cause their degradation and/or subject them to thermal cycles. For example, it is common to lubricate and cool the components of operating equipment by wetting them with an oil or lubricant. As it carries out these functions, the oil or lubricant experiences various environmental stresses that cause its basestock to undergo thermal-oxidative degradation.
Oils are also used as transmission fluids and in hydraulic systems. In these uses, the oil is subjected to pressures, frequent movement, and heat. These stresses also degrade the oil.
Cooking oils are another type of oil that undergo severe thermal-oxidative stresses. The degradation of the basestock can lead to the production of aldehydes and acids, such as carboxylic acids, within the oil which affect the taste of the food.
Because of this degradation, antioxidants are frequently added to oils, lubricants and other fluids to protect their characteristics. As long as the antioxidant system remains intact, the oxidative degradation of the basestock is minimal, and so are changes in the properties.
The antioxidants in oils, lubricants and other fluids are gradually depleted over time. Eventually, the antioxidants become ineffective, allowing large changes in the physical properties of the basestock to occur. Oxidation initiators, such as peroxides, degrade to aldehydes, as in fuels and foods. The thermal input typically experienced by oils, lubricants and other fluids further cause the aldehydes in the oils, lubricants and other fluids to produce carboxylic acids, and also cause the production of phenols. At that point, the oils, lubricants and other fluids are no longer usable to protect equipment, or be consumed, and their useful lives are over. The use of an oil as a lubricant or other fluid in this condition can result in excessive component wear and eventual equipment failure.
It is undesirable to use fuels, food products, and oils, lubricants and other organic materials beyond their useful life. However, as a result of conservative precautionary measures, fuels left in storage may require premature re-refining, reprocessing or disposal. Milk and other food products may be disposed of prematurely as a safeguard, even when still consumable. Oils, for example, lubricants, have scheduled lubricant changes for various types of equipment. The length of operating time between scheduled changes is chosen very conservatively so that lubricant which is beyond its useful life does not remain in the equipment. However, these approaches result in discarding fuels, food products, oils, lubricants and other fluids which still have useful lives.
Early detection of oxidation products is important in monitoring the continued viability of fuels, food products, and oils, lubricants and other organic materials. The ability to analyze fuels, food products, and oils, lubricants and other organic materials for the presence of oxidation products, separately from, or in addition to analyzing for antioxidant depletion, would eliminate the need to reprocess or dispose of fuels, dispose of food products, or change oils on the basis of a fixed schedule. This would allow more efficient use of fuels and food products, and longer and more efficient use of oils, lubricants and other fluids, thereby providing savings in material and labor costs.
Various chemical tests and other methods have been used in the past to evaluate the oxidative degradation of fuels, food products, and oils, lubricants and other organic materials. For example, fuels are typically evaluated by subjective color tests or time-consuming chemical tests for peroxide or hydroperoxide. However, such tests, which typically test for peroxide or hydroperoxide content, are tedious, require skilled personnel, use toxic chemicals, and take up to 30 minutes to perform. These measurements are limited to facilities with fully equipped laboratories, and consequently, are not used in the field by storage tank operators, transportation companies, and end-users of the fuel. Thus, deterioration of the fuel can continue undetected, resulting in gum formation and fuel line clogging.
Milk products have been tested chemically for aldehydes, or subjectively for taste. Other foods have been tested chemically for peroxides, hydroperoxides, carboxylic acids, and other products of oxidative degradation.
Oils, lubricants and other fluids have been chemically tested for carboxylic acids. By way of further example, for oils, lubricants and other fluids, various thermal-oxidative and chemical-oxidative stressing techniques are known which permit evaluation of remaining useful life. However, most of these techniques are also unsuitable for routine use. Thermal-oxidative stressing techniques for oils require the use of high temperatures and pressures and relatively long analysis times, about 30 minutes. Chemical-oxidative stressing techniques for oils are difficult in operation, require unstable reagents, and require even longer analysis times, up to two hours.
More recently, new approaches to testing have been developed and disclosed in U.S. Pat. Nos. 4,744,870, 4,764,258 and 5,071,527 to Kauffman. Assigned to the same assignee as the present invention, these patents disclose methods for determining the remaining useful life of oils which are fast, very accurate, easy to operate, and which can be performed with inexpensive equipment. These methods approach the measurement of useful life by measuring the amount of antioxidant remaining in oils, lubricants and other fluids. In these methods, samples are mixed off-line with a solvent, an electrolyte, and either an organic base or a solid substrate, depending on the type of oil, lubricant or fluid to be tested. The sample is placed in an electrolytic cell and subjected to a cyclic voltammetric analysis. The current generated by the antioxidants and other electroactive species during the cyclic voltammetric analysis is measured and recorded. The remaining useful life for the lubricant is then determined from the oxidation or reduction wave height.
The methods of the U.S. Pat. Nos. 4,744,870 and 4,764,258 patents can only be performed off-line, and are limited to oils or lubricants containing antioxidants. The methods of the U.S. Pat. No. 5,071,527 patent may be performed off-line or on-line, and may be used to perform analysis of antioxidant depletion, peroxide or hydroperoxide level (oxidation initiators), and carboxylic acid level (final oxidation products), but is also limited to the analysis of oils, lubricants and fluids.
In sum, these rapid, straightforward, and more recent methods are able to measure antioxidant depletion prior to oxidative degradation, and some final oxidation products produced by thermal-oxidative processes experienced during use. However, these methods lack the sensitivity to detect very low antioxidant levels and are unable to measure oxidation products of a wide range of stored materials in the early stages of oxidative degradation, while such materials are still usable or consumable.
Therefore, there remains a need for a method which can be used to quickly and accurately test for the initial oxidation products of a wide range of materials including fuels, food products, as well as oils, lubricants and fluids, to determine the freshness of the materials after storage, and to determine if continued use of used materials is possible.